Ultraviolet radiation has become a cause of the deterioration of a number of materials such as a resin and rubber and is said to, for human beings, possibly act as a cause of not only sun tanning or sunburn but also an aging phenomenon or skin cancer. Therefore, an ultraviolet ray-screening agent is widely used in the fields of films, paints, cosmetics, and the like.
As the ultraviolet ray-screening agent, organic ultraviolet ray-screening agents such as a benzophenone-based ultraviolet ray-screening agent, a methoxycinnamic acid-based ultraviolet ray-screening agent, and a dibenzoylmethane-based ultraviolet ray-screening agent or inorganic ultraviolet ray-screening agents such as zinc oxide and titanium oxide are generally used.
The organic ultraviolet ray-screening agents have problems in that there is a concern that the organic ultraviolet ray-screening agents may deteriorate due to heat or the prolonged exposure to ultraviolet radiation, and a single kind of organic ultraviolet ray-screening agent is not capable of absorbing ultraviolet rays in a wide range and thus it is necessary to use a combination of multiple kinds of organic ultraviolet ray-screening agent having different ultraviolet ray absorption wavelengths.
On the other hand, the inorganic ultraviolet ray-screening agents have an effect of absorbing ultraviolet rays having wavelengths that correspond to the band gaps of inorganic particles included in the inorganic ultraviolet ray-screening agent and advantages that the inorganic ultraviolet ray-screening agents do not deteriorate due to heat or the prolonged exposure to ultraviolet radiation, have excellent weather resistance, heat resistance, and the like, and are capable of screening ultraviolet rays in a wide wavelength range since the inorganic ultraviolet ray-screening agents screen ultraviolet rays through scattering attributed to the refractive index of the inorganic particles.
By the way, since the inorganic ultraviolet ray-screening agents scatter not only ultraviolet rays but also visible light rays, even the inorganic ultraviolet ray-screening agents having the above-described advantages have a problem in that the skin easily becomes whitish when a large amount of the inorganic ultraviolet ray-screening agent is blended into a cosmetic or the like. Therefore, in order to cope with the above-described problem, an appropriate combination of the inorganic ultraviolet ray-screening agent and the organic ultraviolet ray-screening agent is used.
As the inorganic ultraviolet ray-screening agent, titanium oxide, zinc oxide, and the like are generally used. Particularly, zinc oxide is capable of screening ultraviolet rays in a wide wavelength range from the UV-A region (320 nm to 400 nm) to the UV-B region (280 nm to 320 nm).
For example, when zinc oxide and titanium oxide are compared with each other in terms of the photocatalytic activity through which a substance in contact with the surfaces of particles is oxidized, zinc oxide has extremely lower photocatalytic activity. In addition, the refractive index of zinc oxide is 2.0, which is lower than the refractive index of titanium oxide (2.7), and thus, in a case in which zinc oxide is made into nanoparticles, the zinc oxide particles have excellent transparency. As a result, zinc oxide has been attracting attention as an ultraviolet ray-screening agent.
Meanwhile, since zinc is an amphoteric element, zinc oxide, which is an oxide of zinc, has characteristics of easily dissolving in an acid and an alkali, in addition, slightly dissolving in water as well, and releasing zinc ions, and these characteristics prevent zinc from becoming a sufficiently stable element.
In addition, while zinc oxide has an extremely lower photocatalytic activity compared with titanium oxide, the photocatalytic activity is desirably suppressed. For example, in a case in which zinc oxide is made into nanoparticles so that the average particle diameter thereof reaches 50 nm or smaller, the specific surface area is increased and thus the photocatalytic activity becomes high.
As described above, zinc oxide particles have particularly significant problems of the release of zinc ions and a high photocatalytic activity.
In addition, compared with oil-based cosmetic products, water-based cosmetic products are not sticky and are capable of obtaining a fresh feeling, and thus, in recent years, the water-based cosmetic products have been used as a variety of cosmetics such as sunscreens, emulsions, and creams. In a case in which zinc oxide is used for the water-based cosmetic, zinc ions being eluted react with a water-soluble macromolecule of an organic ultraviolet ray-screening agent or a viscosity improver, and there is a concern that problems of the degradation of performance as a cosmetic, discoloration, a change in the viscosity, and the like may be caused. Therefore, there has been a problem in that the degree of freedom in formulation is limited.
For example, when a carbomer (carboxy vinyl polymer), which is generally used as a viscosity improver, and zinc oxide are jointly used, a zinc ion being eluted and a carboxylate group (COO—) of the carbomer react with each other, and thus the gel structure of the carbomer breaks, and there is a problem in that the viscosity decreases.
As described above, in order to solve the problems of zinc oxide, there have been a variety of proposals regarding zinc oxide coated with an inorganic oxide.
For example, there has been a proposal regarding a method in which zinc oxide is added to an aqueous solution of silicate of soda so as to be brought into a suspended state, and then the hydrogen-ion exponent (pH) is held at approximately 7, thereby obtaining silica-coated zinc oxide (Patent Literature No. 1).
In addition, there has been a proposal regarding a method in which zinc oxide, which is a raw material, is brought into contact with a composition for forming a silica coating containing silicic acid or a precursor capable of generating silicic acid, which does not contain an organic group and a halogen, water, an alkali, and an organic solvent, thereby obtaining silica-coated zinc oxide which does not deteriorate by weather due to the photocatalytic activity (Patent Literature No. 2).
In addition, there has been a proposal regarding a method in which zinc oxide powder is coated with at least one of organopolysiloxanes and silicone compounds (excluding silane compounds) in a non-gaseous state, and is fired at a temperature in a range of 600° C. to 950° C. in an oxidizing atmosphere, thereby obtaining activity-suppressing zinc oxide powder coated with silicon oxide (Patent Literature No. 3).